1. Technical Field
The present disclosure generally relates to the conversion of starch to simpler carbohydrates, and more particularly to apparatus and methods for converting starch to dextrose. More specifically, the disclosure relates to the reduction of mass transfer limitations for the hydrolysis of starch.
2. Background of the Invention
Starch can be hydrolyzed into simpler carbohydrates by acids, various enzymes, or a combination of the two. There are many food products synthesized by the hydrolysis of starch. The extent of conversion is typically quantified by dextrose equivalent (DE), which is roughly the fraction of the glycoside bonds in starch that have been broken. Maltodextrin is a lightly hydrolyzed (DE 10-20) starch product used as a bland-tasting filler and thickener. Viscous solutions of various corn syrups (DE 30-70) are used as sweeteners and thickeners in many kinds of processed foods. Dextrose (DE 100), or commercial glucose (D-glucose), is prepared by the complete hydrolysis of starch. High fructose syrup is obtained by treating dextrose solutions with the enzyme glucose isomerase, until a substantial fraction of the glucose has been converted to fructose.
In the United States, dextrose and high fructose corn syrup are particularly important commercial food products. Dextrose is a white odorless tasteless granular or powdery complex carbohydrate having the chemical formula (C6H10O5)x. Dextrose is the chief form of carbohydrate storage in plants and has additional applications in adhesives, laundering, pharmaceuticals, and medicine. High fructose corn syrup is the principal sweetener used in sweetened beverages to lower the cost of production. A lower quantity of the high fructose corn syrup can be used in recipes compared to glucose because fructose tastes sweeter than glucose.
Historically, the starch wet-milling industry produced all starch-derived syrups by acid hydrolysis. There are several disadvantages of the acid process that were corrected by replacing the acid process with a two-step process as disclosed in U.S. Pat. No. 2,891,869. In the disclosure, the first step comprises solubilizing or liquefying refined raw starch to create a low DE syrup product. Liquefication is accomplished by limited hydrolysis at high temperature using either acid or thermostable endoamylases, such as those produced by Bacillus lichenformis. The second step comprises subjecting the low DE syrups produced in the first step to more extensive hydrolysis reactions. The second step may also be known as saccharification. The second step produces syrups consisting of low molecular weight sweet sugars using enzymes that are very specific with regard to the products they form. The overall procedure is thus referred to as an acid-enzyme or a double enzyme process depending on the mode of liquefaction.
The process of breaking a complex carbohydrate, such as starch or cellulose, into its monosaccharide components is also referred to as saccharification. U.S. Pat. No. 2,891,869 discloses the preparation of cornstarch derived syrups using the acid-enzyme process. In the disclosure, syrups of varying composition were prepared by altering the saccharifying enzymes utilized. The patent discloses that fungal glucoamylase (GA) produces glucose as the sole product and that malt diastase produces the disaccharide maltose as a major product. Syrups containing various proportions of these two sugars may be prepared by saccharifying the substrate with a combination of glucoamylase and malt diastase. Subsequent investigations have been concerned with the development of enzyme systems that increase the degree of starch saccharification and thereby the yields of these products.
A number of procedures covering immobilized enzyme technology for continuous dextrose production from starch have been described. In the disclosures regarding enzyme immobilization, immobilization of the enzyme glucoamylase has been the focus. Many methods of glucoamylase immobilization are available, for example, the methods described in U.S. Pat. Nos. 2,717,852; 3,519,538; 3,619,371; 3,627,638; 3,672,955; 3,715,277; 2,783,101; and 3,950,222.
Accordingly, there is a need in the industry for improved methods of producing dextrose and other starch hydrolysates from starch, whereby production rates are increased, improved reactant mixing, and lower reactant requirements are commercially feasible.